require 'string/scrub' if RUBY_VERSION < '2.1'

`string-scrub` is specified in gemspec, but is not required in code.

So, I added `require` clause, and it works in my td-agent with ruby 1.9.3.
Which do you think better to determine ruby version?

* if RUBY_VERSION < '2.1'
* if RUBY_VERSION.to_f < 2.1